Direct Write Sensors

ABSTRACT

A method of making an acoustic wave sensor includes the steps of providing a piezoelectric substrate layer and printing on the substrate layer a sensor layer comprising a first interdigitated acoustic wave transducer, a sensing film, and positioned on an opposing side of the sensing film from the first interdigitated acoustic wave transducer at least one selected from the group consisting of a second interdigitated acoustic wave transducer and a Bragg reflector. An insulation layer can be printed. An antenna can be printed in an antenna layer, and the insulation layer can be interposed between the antenna layer and the sensor layer. An electrical connection can be printed between the antenna and the first interdigitated acoustic wave transducer. An acoustic wave sensor is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a divisional of U.S. patent application Ser. No.15/246,907, filed Aug. 25, 2016, which claims priority to U.S. PatentApplication No. 62/214,233, filed Sep. 4, 2015, entitled “Direct WriteSensors”, the entireties of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contract No.DE-AC05-00OR22725 awarded by the US Department of Energy. The governmenthas certain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to acoustic wave sensors, and moreparticularly to methods and designs for making acoustic wave sensors.

BACKGROUND OF THE INVENTION

Bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices havebeen fabricated for decades using traditional semiconductor integratedcircuit methods. These methods are mature and optimized to produce >1billion high fidelity, matched filters for RF and microwavecommunications such as cell phones, walkie-talkies, and other devices.However, relatively sophisticated and expensive equipment is required toproduce these devices.

High performance acoustic wave structures have recently been exploitedfor sensor applications. Acoustic wave structures have been demonstratedto perform as temperature, strain and hydrogen sensing devices,employing traditional photo-lithography for device fabrication.Photo-lithography is time consuming and comparatively expensive, andaccordingly new methodologies for manufacturing such sensors arenecessary.

SUMMARY OF THE INVENTION

A method of making an acoustic wave sensor includes the steps ofproviding a piezoelectric substrate layer and printing on the substratelayer a sensor layer comprising a first interdigitated acoustic wavetransducer, and at least one other feature selected from the groupconsisting of a sensing film, an interdigitated acoustic wavetransducer, and a Bragg reflector. A second interdigitated acoustic wavetransducer or one or more Bragg reflectors can be positioned on anopposing side of the sensing film, if present, from the firstinterdigitated acoustic wave transducer. An insulation layer can beprinted. An antenna can be printed in an antenna layer, and theinsulation layer can be interposed between the antenna layer and thesensor layer. An electrical connection can be printed between theantenna and the first interdigitated acoustic wave transducer. Theprinting method can be performed by aerosol jet direct digital printing.

The acoustic wave sensor can be a bulk acoustic wave sensor. The Qfactor of the bulk acoustic wave sensor can be greater than 1000.

The piezoelectric substrate can be provided as a film and moved roll toroll during the printing process.

The method can further include the step of controlling the movement ofthe aerosol jet thorough a control system and at least one processor.

A plurality of acoustic wave sensors can be printed on the piezoelectricsubstrate, and the method can further include the step of separating thesubstrate and the acoustic wave sensors into individual acoustic wavesensors.

The sensing film can include a hydrophilic material. The sensing filmcan include palladium. The sensing film can include graphene. Thesensing film can include a carbon nanotube array. The sensor can have amaximum dimension of less than 2 mm².

The sensor layer and antenna layer can be printed on opposing sides ofthe piezoelectric substrate layer. At least a portion of the sensorlayer and the antenna layer can be printed simultaneously.

An acoustic wave sensor can include a piezoelectric substrate layer anda sensor layer joined to the substrate layer and comprising a firstinterdigitated acoustic wave transducer, and at least one other featureselected from the group consisting of a sensing film, an interdigitatedacoustic wave transducer, and a Bragg reflector. A second interdigitatedacoustic wave transducer or one or more Bragg reflectors can bepositioned on an opposing side of the sensing film, if present, from thefirst interdigitated acoustic wave transducer. All of the interdigitatedacoustic wave transducers and Bragg reflector can be aerosol jetprinted.

An antenna layer can be provided and an insulation layer can beinterposed between the antenna layer and the sensor layer, and theinsulation layer and antenna layer can be aerosol jet printed. At leastone of the sensor layer and antenna layer can include a dielectricmatrix material. The sensor layer and the antenna layer can be printedon opposing sides of the piezoelectric substrate layer.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a schematic diagram of a surface acoustic wave deviceaccording to the invention.

FIG. 2 is a schematic diagram of a surface acoustic wave devicefunctionalized by a chemical specific film.

FIG. 3 is a schematic diagram of aerosol jet printing of surfaceacoustic wave devices on a piezoelectric wafer.

FIG. 4 is a schematic diagram of multi-head printing of surface acousticwave devices.

FIG. 5 is a schematic diagram of multi-head printing on both top andbottom surfaces of moving media.

FIG. 6 is a schematic diagram illustrating pulsed thermal processing ofprinted surface acoustic wave structures.

FIG. 7A is a schematic diagram of standard aerosol jet printing and FIG.7B is a schematic diagram of aerosol jet printing augmented byelectrostatic/electromagnetic focusing.

FIG. 8 is a perspective schematic view of a surface acoustic wavedevice.

FIG. 9 is a cross-section of the surface acoustic wave device.

FIG. 10 is a perspective schematic view of a surface acoustic wavedevice with functionalizing film.

FIG. 11 is a cross-section.

FIG. 12 is a schematic cross-section of a surface acoustic wave devicewith printed fractal antenna.

FIG. 13 is a bottom view.

FIG. 14 is a schematic cross-section of the surface acoustic wave devicewith a printed spiral antenna.

FIG. 15 is a bottom view.

FIG. 16 is a schematic cross-section of a bulk acoustic wave deviceshowing an acoustic wave propagating through the piezoelectricsubstrate, and with a functionalizing film on top of device.

FIG. 17 is a schematic cross-section of a multi-layer bulk acoustic wavedevice and a string of reflected signals.

FIG. 18 is a schematic cross-section of a variable layer thickness bulkacoustic wave device providing both frequency and time diversity inreflected signals.

FIG. 19 is a schematic diagram of an interdigitated transducer launchingan acoustic wave.

FIG. 20 is a schematic diagram of a Bragg reflector returning anacoustic signal to the interdigitated transducer.

FIG. 21 is a schematic diagram of an interdigitated transducer and Braggreflector and launched acoustic wave and reflected acoustic wave signal.

FIG. 22 is a schematic diagram and photograph of corresponding printedelements in a Bragg reflector.

FIG. 23 is a photograph of printed interdigitated transducer electrodes.

FIG. 24 is a schematic diagram of a dual channel surface acoustic wavedevice.

FIG. 25 is a schematic cross-section of a dual channel surface acousticwave device showing antenna connections.

FIG. 26 is a schematic diagram of a dual channel surface acoustic wavedevice functionalized by hydrogel for moisture sensing.

FIG. 27 is a schematic diagram of a dual channel surface acoustic wavedevice functionalized by a Palladium film for hydrogen sensing.

FIG. 28 is a schematic diagram of a dual channel surface acoustic wavedevice functionalized by graphene nanostructures for methane detection.

FIG. 29 is a schematic diagram of the dual channel surface acoustic wavedevice functionalized by carbon nanotubes for CO₂ detection.

FIG. 30 is a photograph of printed dual channel surface acoustic wavedevices.

FIG. 31 is a schematic diagram illustrating an array of surface acousticwave devices printed simultaneously and sequentially.

FIG. 32 is a plot of sheet resistance (ohms/Sq) versus annealingtemperature (° C.) for printed structures by an ultrasonic atomizer.

FIG. 33 is a plot of sheet resistance (ohms/Sq) versus annealingtemperature (° C.) for printed structures by a pneumatic atomizer.

FIG. 34 is a photograph of a 10 μm line printed on a piezoelectricpolymer (PV DF) substrate with 50 nm silver particles in a liquidsuspension.

FIG. 35 is a schematic diagram of an ultrasonic atomizer print head.

FIG. 36 is a schematic diagram of a pneumatic atomizer print head.

DETAILED DESCRIPTION OF THE INVENTION

A method of making an acoustic wave sensor includes the steps ofproviding a piezoelectric substrate layer and printing on the substratelayer a sensor layer comprising a first interdigitated acoustic wavetransducer, and at least one other feature selected from the groupconsisting of a sensing film, an interdigitated acoustic wavetransducer, and a Bragg reflector. A second interdigitated acoustic wavetransducer or one or more Bragg reflectors can be positioned on anopposing side of the sensing film, if present, from the firstinterdigitated acoustic wave transducer. An insulation layer can beprinted. An antenna can be printed in an antenna layer, and theinsulation layer can be interposed between the antenna layer and thesensor layer. The antenna can alternatively be provided separately, andprinted or produced by other means. An electrical connection can beprinted between the antenna and the first interdigitated acoustic wavetransducer. The printing method can be performed by aerosol jet directdigital printing.

The movement of the aerosol jet thorough a control system and at leastone processor. Any suitable control mechanism and processor orcontroller is possible.

A plurality of acoustic wave sensors can be printed on the piezoelectricsubstrate, and the method can further include the step of separating thesubstrate and the acoustic wave sensors into individual acoustic wavesensors. The individual sensors can be cut by any suitable device andpackaged as individual sensors or groups of sensors.

The sensing film can be any suitable material. The sensing film can forexample include a hydrophilic material. The sensing film can includepalladium. The sensing film can include graphene. The sensing film caninclude a carbon nanotube array. The sensor can have a maximum dimensionof less than 2 mm². The acoustic wave sensor can be printed without asensor film to measure temperature, because the piezoelectric materialalone can be temperature sensitive in the desired temperature range.

The sensor layer and antenna layer can be printed on opposing sides ofthe piezoelectric substrate layer. The antenna can connect to more thanone sensor. Sensors can be printed on both sides of the piezoelectricsubstrate layer. Vias can be created through the substrate and suitableelectrical connections can be provided through the vias by any suitablemethod, including aerosol jet printing. At least a portion of the sensorlayer and the antenna layer can be printed simultaneously.

An acoustic wave sensor can include a piezoelectric substrate layer anda sensor layer joined to the substrate layer and comprising a firstinterdigitated acoustic wave transducer, and at least one other featureselected from the group consisting of a sensing film, an interdigitatedacoustic wave transducer, and a Bragg reflector. A second interdigitatedacoustic wave transducer or one or more Bragg reflectors can bepositioned on an opposing side of the sensing film, if present, from thefirst interdigitated acoustic wave transducer. All of the interdigitatedacoustic wave transducers and Bragg reflectors can be aerosol jetprinted.

An antenna layer can be provided and an insulation layer can beinterposed between the antenna layer and the sensor layer, and theinsulation layer and antenna layer can be aerosol jet printed. At leastone of the sensor layer and antenna layer can include a dielectricmatrix material. The sensor layer and the antenna layer can be printedon opposing sides of the piezoelectric substrate layer. Many antennadesigns can be utilized including, without limitation, dipole antennas,gap loaded Archimedean spiral antennas, fractal antennas, and patchantenna arrays.

FIG. 1 is a schematic diagram of a surface acoustic wave device 10according to the invention. The device 10 includes a piezoelectricsubstrate 14. An interdigitated transducer 18 has interdigitatedtransducer lines 26 and contacts 34 which connect to antenna 42. Aninterdigitated transducer 22 has interdigitated transducer lines 30 andcontacts 38 which connect to antenna 46.

FIG. 2 is a schematic diagram of the surface acoustic wave device 10functionalized by a chemical specific film 50. The film 50 can beprinted by aerosol jet printing. The film 50 can also be applied byother methods.

FIG. 3 is a schematic diagram of aerosol jet printing of surfaceacoustic wave devices on a piezoelectric wafer. An aerosol jet printhead 54 includes an outer housing 58 and neck 62. An inner sheath 66defines an annular space 74 for the flow of sheath gas that serves tofocus and direct the aerosol 74 into a fine jet 78. Movement of theprint head 54 can be controlled by a processor and control mechanisms todirect the jet 78 so as to create sensors 82 on the substrate 86. Thesubstrate 86 can be in the form of a wafer and can be moved in thedirection of arrows 90 and 94 to permit the printing of a multitude ofsensors on the same wafer.

FIG. 4 is a schematic diagram of multi-head printing of surface acousticwave devices. This is shown schematically by print head 54A which movesaccording to direction arrow 104, and print head 54B which can move inthe same or a different direction as shown by arrow 108. Accordinglymultiple sensors 100 can be printed rapidly and in some casessimultaneously onto substrate 98.

FIG. 5 is a schematic diagram of multi-head printing on both top andbottom surfaces of moving media. Multiple print heads such as printheads 54A and 54B can be provided to print sensors 120 onto substrate116. The substrate 116 can be in the form of a film which moves fromroll 124 to roll 128 in a roll-to-roll printing process. A print head112 can be provided on an underside of the substrate 116 such thatsensors can be printed simultaneously on both sides of the substrate116, or components of sensors can be printed on opposing sides of thesubstrate 116, such as sensor layers on one side and antenna layers onthe opposing side. The substrate 116 can be moved in the direction ofarrow 132 such that a succession of sensors 120 can be rapidly printed.The process can also be configured to move vertically from roll to roll.

FIG. 6 is a schematic diagram illustrating pulsed thermal processingdevices 136 and 144 emitting high intensity radiation 140 and 148respectively to effect curing of the sensor layers on the substrate 116.

FIG. 7 is a schematic diagram of (FIG. 7A) of a standard aerosol jetprint head and (FIG. 7B) aerosol jet printing augmented byelectrostatic/electromagnetic focusing. The focusing can be by anysuitable structure such as electromagnets 152, 156 which focus the jet78 to a very fine micro jet 160 enabling printing of features less than10 μm in extent.

FIG. 8 is a perspective schematic view of a surface acoustic wave device180 having interdigitated transducers 172 and 176 printed onpiezoelectric substrate 168. FIG. 9 is a cross-section of the surfaceacoustic wave device 180.

FIG. 10 is a perspective schematic view of the surface acoustic wavedevice 168 having substrate 180 with interdigitated transducers 172 and176, and with functionalizing film 184. The functionalizing film 184 canalso be applied by aerosol jet printing. As seen in FIG. 11, thefeatures can be printed in a single pass of the substrate 180 past theprint heads.

FIGS. 12-13 are a schematic cross-section and bottom view of a surfaceacoustic wave device 190 in which a substrate 194 has interdigitatedtransducers 198 and 202 printed on a 1^(st) side 206. A 2^(nd) side 210of the substrate 194 has printed thereon antennas 214 and 218. Multiplesensors can be connected to the same antenna. The antenna 214 can havecontact 222 and the antenna 218 can have contact 226 which contacts canbe printed with the aerosol jet printer on a bottom side 210 of thesubstrate 194.

FIGS. 14-15 are a schematic cross-section and bottom view of the surfaceacoustic wave device 230 with a having a substrate 234. Interdigitatedtransducers 238 and 240, and sensor film 244 can be printed on a 1^(st)side 248 of the substrate 234. On a 2^(nd) side 252 of the substrate 234can be printed spiral antennas 256 and 260. Many different antennadesigns can be printed.

FIG. 16 is a schematic cross-section of a bulk acoustic wave device 264having a substrate 268. An interdigitated transducer 272 is provided ona 1^(st) side 276 of substrate 268. An interdigitated transducer 280 isprovided on 2^(nd) side 284 of substrate 268. A functionalizing film 290is shown on top of the 1^(st) interdigitated transducer 272. An acousticwave 294 is shown propagating through the piezoelectric substrate 268.The interdigitated transducers 272 and 280, as well as functionalizingfilm 290 can be printed on the substrate 268 by aerosol jet printing.The Q factor of the bulk acoustic wave sensor can be greater than 1000.

FIG. 17 is a schematic cross-section of a multi-layer bulk acoustic wavedevice 300 having a substrate 304 comprised of several layers 304 A-E. A1^(st) interdigitated transducer 308 can be provided on a 1^(st) side312 of the substrate 304. A 2^(nd) interdigitated transducer 316 can beprovided on 2^(nd) side 320 of substrate 304. A functionalizing film 324can be provided on top of the 1^(st) interdigitated transducer 308. Theacoustic waves indicated by arrows 328 A-E can produce a string ofreflected signals. All features of the device 300 can be printed on thesubstrate 304 by aerosol jet printing.

FIG. 18 is a schematic cross-section of a variable layer thickness bulkacoustic wave device 340 having a substrate 344. The substrate 344 iscomprised of layers 348, 352, and 356 of variable layer thickness. A1^(st) interdigitated transducer 360 is provided on a surface 364 of thesubstrate 344. A 2^(nd) interdigitated transducer 368 can be provided ona side 372 of the substrate 344. A functionalizing film 376 can beprovided on top of the 1^(st) interdigitated transducer 360. Acousticsignals 380, 384, 388 will be reflected by the variable thickness layersaccording to the wavelengths of the signal and the properties andthickness of the layers 348, 352, and 356. The interdigitatedtransducers 360 and 368 and functionalizing film 376 can be printed byaerosol jet printing. The variable thickness bulk acoustic wave device340 provides both frequency and time diversity in reflected signals.

FIG. 19 is a schematic diagram of an interdigitated transducer 392having contacts 396 and 400 and interdigitated lines 404. An acousticwave schematically indicated as 408 is propagated in the direction shownby arrow 412. FIG. 20 is a schematic diagram of a Bragg reflector 416having ends 420 and reflecting layers 424. The Bragg reflector 416selectively reflects signal 428 in the direction of arrow 432. As shownin FIG. 21, the acoustic signal 408 is emitted and returned to theinterdigitated transducer 392 as reflected signal 428. As shown in FIG.22 the Bragg reflector 416 can be printed on substrate 430 by aerosoljet printing. There is shown in FIG. 23 printed electrodes 434 and 438of a dual channel interdigitated transducer printed on substrate 442. Aseries of Bragg reflectors can also be used to create an identificationcode. Each Bragg reflector has a unique geometry (different line widthand spacing) causing a group of returned signals each having a differentfrequency.

FIGS. 24-25 are a schematic diagram and cross-section of a dual channelsurface acoustic wave device 450. The device 450 includes a 1^(st)electrode 454 having branches 458 and 462. A 2^(nd) electrode 466 has aterminus 470. Interdigitated transducer lines 474 and 478 are positionedbetween the 1^(st) branch 458 and the terminus 470 and the 2^(nd) branch462 and the terminus 470. An antenna 490 can connect across theelectrodes 454 and 466 to impart a signal. The antenna 490 can beconnected by line 494 to electrode 466, and by line 498 to electrode454.

FIG. 26 is a schematic diagram of the dual channel surface acoustic wavedevice 450 functionalized by hydrogel 502 for moisture sensing. FIG. 27is a schematic diagram of the dual channel surface acoustic wave device450 functionalized by a Palladium film 506 for hydrogen sensing. FIG. 28is a schematic diagram of the dual channel surface acoustic wave device450 functionalized by graphene nanostructures 510 for methane detection.FIG. 29 is a schematic diagram of the dual channel surface acoustic wavedevice functionalized by carbon nanotubes 514 for CO₂ detection. Thesefilms can be printed by aerosol jet printing or fabricated by othermeans. Functionalization can also be achieved with organic compoundscalled amines. Amines are derived from ammonia and can be tailored forCO₂ detection, for example. The amine is also printed by aerosol jetmethods.

FIG. 30 is a photograph of printed dual channel surface acoustic wavedevices 518 printed according to the invention. FIG. 31 is a schematicdiagram illustrating how an array of surface acoustic wave devices 522can be printed simultaneously and sequentially on a single substrate.Employing a printing process that implements a 2 mm² device with 2 mmpitch, results in ˜2,000 devices/wafer. A 50% decrease in sensor sizemore than triples the number of sensors per wafer. Multiple acousticwave sensors as in FIG. 30 can be provided for sensing differentphenomena, and can be printed and combined to determine conditions orproperties not able to be determined with a single acoustic wave sensor(such as to measure temperature and humidity in combination to determinedew point). An array of sensors such as shown in FIG. 31 can be used tomeasure the same properties, and the results combined to create astatistical result with improved accuracy over the result from a singlesensor.

FIG. 32 is a plot of sheet resistance (ohms/Sq) versus annealingtemperature (° C.) for structures printed by an ultrasonic atomizer. Theplot shows that sheet resistance generally decreases with increasingannealing temperature. FIG. 33 is a plot of sheet resistance (ohms/Sq)versus annealing temperature (° C.) for structures printed by apneumatic atomizer. The decrease in sheet resistance with annealing isnot as pronounced for the pneumatic atomizer as for the ultrasonicatomizer.

FIG. 34 is a photograph of a 10 μm line printed with 50 nm silverparticles suspended in alcohol on a piezoelectric polymer (PVDF)substrate. This figure illustrates the very fine features that can beprinted for sensors according to the invention.

FIG. 35 is a schematic diagram of an ultrasonic atomizer print headassembly 520. The print head 520 includes an ultrasonic transducer 524for applying energy to the ink bath 528. This creates droplets 522 whichmix with gas from gas inlet 544 in chamber 536 and provide an aerosol inconduit 540 which exits at outlet 548 to the print head. The transducer524 produces high-frequency pressure waves causing the ink or otherliquid source material to be atomized. The atomized droplets are thenentrained in the gas stream and deposited on the substrate. This devicecan be used for materials with viscosity in the range of 1-5 cP andparticulate suspensions of 50 nm or less.

FIG. 36 is a schematic diagram of a pneumatic atomizer print head 560.Gas enters the device through gas inlet 564 and mixes with in bath 568.The aerosol so created exits at outlet 572 to the print nozzle. Anexhaust 576 can also be provided. The pneumatic atomizer useshigh-velocity gas stream to shear liquid ink into small droplets (1 to 5μm). Compressed gas is expanded through the atomizer nozzle, producing ahigh-velocity jet. Larger droplets impact the walls, returning to thereservoir. Smaller droplets remain suspended for deposition. Thepneumatic atomizer can be used for inks, pastes, or other materials withviscosity in the range of 1 to 1000 cP and particulate suspensions <500nm in size.

The BAW/SAW sensors of the invention can be fabricated by suitableaerosol jet equipment, such as can be obtained from Optomec, Inc.(Albuquerque, N. Mex.). Other possible fabrication methods including inkjet printing, screen printing, syringe printing, plasma pray printing,and myriad fused deposition printing methods including laser andelectron beam melting, are capable of printing structures that fall intoa subset of aerosol jet capabilities. Aerosol jet is a highly versatileadditive manufacturing tool capable of efficiently depositing metals,semi-conductors, insulators, polymers and even biological materials inprecise miniature patterns or larger structures depending upon thesystem configuration.

SAW devices rely on precise feature size printing in order to functionin recognized ISM communication frequency bands compatible with manyexisting process control and automation systems. The invention hasachieved some of the smallest aerosol jet sensor structures to date.Both a dual channel SAW device and a Bragg reflector having ˜6 μmfeatures have been produced. This feature size produces an operatingfrequency range of ˜125-175 MHz in the resulting SAW device. Controllingthe operating frequency of acoustic wave sensors is important for tworeasons: a) the operating frequency largely determines the operationalcommunication distance between the passive sensor and the interrogator;and b) tailoring the frequency response provides each sensor with aunique identity.

Fabricating BAW sensors also requires precise deposition control. BAWdevices rely on precise control of device layer thickness. Aerosol jetprovides ˜10 nanometer layer thickness printing control. The performanceof the device, commonly represented by the Q-factor, is also highlydependent on the device geometry. Aerosol jet printed BAW devices canachieve Qf>1,000, enabling detection limits as small as 1 part in 10⁹ (1part/billion or 1 ppb). Similarly, SAW devices have also demonstratedsub-ppm detection limits.

The invention is capable of producing sensors with many different modesof sensing including: physical sensors, chemical sensors, biologicalsensors, location and tracking sensors, and tamper resistant seals.Possible sensor modalities include, without limitation physical sensorsfor temperature, strain, pressure, corrosion. Chemical sensors are usedto measure humidity/moisture, CO₂, CH₄, HF, VOCs, H₂, and the like.Electronic sensors measure current flow, voltage and charge. Biologicalsensors test for food spoilage, bacterial and fungal agents, blood sugarlevel, and other biometric factors. Location and tracking sensors andtamper resistant seals are also becoming more common. Sensingapplications include physical sensing for energy control, conditionmonitoring, and presence detection. Electronic sensing is performed forgrid monitoring and surveillance. Chemical sensing is used for pipelinemonitoring, leak/plume detection, and accident response. Multiple sensormodalities can easily be combined to produce a unique signature orfinger print. Other applications are possible.

BAW and SAW sensor functionality is realized by measuring the change inthe acoustic wave propagation velocity within the device substrate. Thefollowing relationship describes this phenomenon by relating theinterrogator operating frequency to the acoustic propagation velocitydivided by the acoustic wavelength. The physical feature size of thedevice (BAW—layer thickness and SAW—spatial feature size and pitch)largely determines the operating parameters.

v=c/λ  (1)

In equation (1), v represents the electromagnetic interrogationfrequency as described above. The invention is capable of printing SAWdevices with 6-20 μm feature size and operational frequencies rangingfrom 37.5-175 MHz. The device propagation velocity represented by c inequation (1) is the acoustic propagation velocity within the devicesubstrate, not the speed of light. The acoustic velocity can range from1,500 to nearly 4,500 m/s, depending upon substrate material selection.The λ represents the acoustic wavelength of the device determined bylayer thickness or surface features for BAW and SAW devicesrespectively. For SAW devices being printed currently, the Braggreflectors and IDT include ˜10 μm line width and pitch. This criticalfeature size is generally interpreted as λ/4, the quarter-wave couplingwavelength from antenna theory.

v=c/(λ/4)=4,000 m/s/(10 μm/4)=˜75 MHz   (2)

In order to create an acoustic sensor, it is necessary to couple to thesensor substrate and modify the acoustic propagation velocity. There aretwo principle ways in which acoustic velocity is altered: a) massloading—the substrate gains additional inertia through increased massloading causing a mechanical impedance to the acoustic wave; and b)strain induced modification of the substrate elastic modulus—theacoustoelectric effect. Both phenomena result in a change of amplitude,phase, frequency or a time delay—the returned signal arrives later intime proportional to the changed mass or strain.

Surface acoustic wave devices require their substrate to be aferroelectric material. More specifically, piezoelectric substratematerials such as lead-zirconate-titanate (PZT), lithium niobate, orquartz produce the greatest response or conversion efficiency. Polymericsubstrates, such as polyvinyladene fluoride (PVDF), also exhibitsferroelectric properties, allowing acoustic devices to be printed uponflexible substrates. Other materials are possible.

Piezoelectric materials exhibit temperature dependent response, hence aSAW device functions as a temperature sensor without furtherfunctionalization. To sense other physical variables, the SAW device isfunctionalized by mechanical means. For instance, a very sensitivepressure sensor is created by functionalizing the SAW device by makingthe substrate a flexible element or diaphragm. This flexing is caused bychanging pressure, creating a changing strain in the substrate that isdetected as an acoustic velocity shift in the SAW device.

Sensing chemical or biological variables requires functionalization in adifferent manner. Humidity sensing is a relatively simple example offunctionalization via mass loading. Polymethylmethacrilate (PMMA), is areversible hydrophilic material that is readily printed by AJ methods.Hydrophilic aerogels can also be printed. FIG. 26 illustrates a SAWhumidity sensor architecture. The hydrophilic region is deposited intoone of the SAW device channels. An acoustic velocity change occurs asmoisture loading changes. As the film increases in mass, the acousticvelocity decreases, and vise versa.

Many film treatments are possible to functionalize the SAW device. Phaseseparated glass and metal oxide structures provide a scaffold havingextremely high surface area. These high surface area structures are thenfunctionalized by creating chemically specific absorption sites, suchas, palladium nanoparticles for hydrogen sensing. A nanostructuredspinodally phase separated ultra-porous silica film can be deposited onlithium niobate.

The sensor design and functionalization methods can be utilized with asingle aerosol jet deposition head or multiple heads. The sensor designscan be scaled-up for roll-to-roll processing by employing a multi-headapproach. Aerosol jet deposition can be performed with a parallel headarray, but additional head arrays can also be performing additionalfabrication steps down stream. A variety of materials can be printed,including conductors, insulators, semi-conductors, dielectrics, andbiological materials. Feature sizes less than ˜10 μm and with no realupper limit are possible. The method is compatible with rigid andflexible substrates. The integration of sensors, antennas, andelectronics is possible, with a wide range of operating communicationsfrequencies

Key controls include ink selection (viscosity, conductor, insulator anddielectric), substrate selection (material, thickness, wafer or polymerfilm, deposition rate, nozzle size, atomization carrier gas pressure andflow rate, sheath gas pressure and flow rate, exhaust gas pressure andflow rate, scanning speed and thermal management.

EXAMPLE

Surface acoustic wave (SAW) sensors were developed using aerosol jetdirect digital printing, employing an Optomec AJ200 system. Singlechannel and dual channel sensors were demonstrated. Sensor modalitiesinclude temperature, humidity and CO₂. Volatile organic compound (VOC)sensors and other physical sensors such as pressure, corrosion, currentand voltage are possible. The invention allows printing features assmall as 5 microns, and feature size to 1 micron or less are possible.In order to fabricate these sensors, several parameters must becontrolled. A 100 micron nozzle, combined with 25 ccm aerosol flow ratefrom an ultrasonic atomizer (UA) was utilized. The UA was set to 0.3 mA,with a nitrogen sheath gas flow rate of 22 ccm. The processing speed was1 mm/sec, depositing Clariant silver colloid 25 ink with on a lithiumniobate substrate. The total print time was 2 minutes.

The ease and low cost of manufacture of sensors created by the inventionpresent the possibility of disposable sensors. Disposable sensors createa paradigm shift in measurement, control, monitoring, etc. by opening upnew methods and approaches. For example, sensors embedded into productsduring manufacturing, sensors that are embedded into the manufacturingdevices/equipment for intimate process monitoring, and sensors thatflow-through the manufacturing process on test articles or the actualproduct itself, can be used to characterize process conditions in situ.Gaining intimate knowledge of manufacturing process conditions has neverbeen done in this way and could lead to significant efficiency gains.Further, having process knowledge, item-by-item has also never beenachieved and could be exploited for efficiency gains. For example, manythousand sheets of drywall are manufactured on a daily basis. Thecondition of the sheet versus the process could be measured providingpinpoint information of actual product condition, versus global processinformation. Disposable RF/SAW sensors could be applied to each sheet ofdrywall, monitoring in real-time, the temperature and moisture contentof the sheet. Process conditions, such as oven temperature, residencetime, etc. are then controlled in response to optimum product conditionsinstead of being controlled to a global fixed point.

There are many advantages of aerosol jet printing versus inkjet andscreen printing. There is a wider possible material range (metals,alloys, resistor paste, dielectrics, polymers, adhesives). Mostmaterials are standard off-the-shelf. There is no customization that isrequired. Significantly higher metal loadings are possible, as much as50%-70 weight percent for high resolution printing. There is a 10 timessmaller nano-particle droplet size to produce finer features. There is20-30 times higher yield than inkjet per nozzle. There are highervolumetric rates and metal loading. There is superior throughput nozzlewith higher material loading per droplet. There is 5 times the distanceabove substrate enables conformal printing on non-planar surfaces. Thereare much finer feature sizes (10 μm) and tighter pitch (20 μm) pitch.There is better edge definition.

There are many advantages of aerosol jet printing versus screenprinting. Noncontact printing eliminates breakage of substrates.Noncontact process enables conformal printing on non-planar substrates.The aerosol jet process is easier to implement changes, and there is nohard tooling. Thinner layer deposits reduced material waste. Much finerfeature size (10 μm) and tighter pitch (20 μm) are possible.

Direct printing of conductive patterns eliminates the use of toxicchemicals and saves costs. Nano silver conductive inks allow directprinting of conductive patterns for electronic devices on any surface.Hazardous chemical metallization processes are replaced by water-basedink containing metal particles to form the electronic pattern.Nano-sized silver particles can be cured at low temperatures to createan electric conductor almost as conductive as a pure silver wire.

The invention is capable of taking alternative forms without departingfrom the spirit or essential attributes thereof, and accordinglyreference should be made to the following claims to determine the scopeof the invention.

We claim:
 1. A method of making an acoustic wave sensor, comprising thesteps of: providing a piezoelectric substrate layer; printing on thesubstrate layer a sensor layer comprising a first interdigitatedacoustic wave transducer, and at least one other feature selected fromthe group consisting of a sensing film, an interdigitated acoustic wavetransducer, and a Bragg reflector.
 2. The method of claim 1, comprisingprinting a sensing film, and printing on an opposing side of the sensingfilm from the first interdigitated acoustic wave transducer at least oneselected from the group consisting of a second interdigitated acousticwave transducer and a Bragg reflector.
 3. The method of claim 1,comprising the steps of; printing an insulation layer; printing anantenna in an antenna layer, wherein the insulation layer is interposedbetween the antenna layer and the sensor layer; and, printing anelectrical connection between the antenna and the first interdigitatedacoustic wave transducer.
 4. The method of claim 1, wherein the printingis by aerosol jet direct digital printing.
 5. The method of claim 1,wherein the acoustic wave sensor is a bulk acoustic wave sensor.
 6. Themethod of claim 5, wherein the Q factor of the bulk acoustic wave sensoris greater than
 1000. 7. The method of claim 1, wherein thepiezoelectric substrate is provided as a film and is moved roll to rollduring the printing process.
 8. The method of claim 1, furthercomprising the step of controlling the movement of the aerosol jetthorough a control system and at least one processor.
 9. The method ofclaim 1, wherein a plurality of acoustic wave sensors are printed on thepiezoelectric substrate, and further comprising the step of separatingthe substrate and the acoustic wave sensors into individual acousticwave sensors.
 10. The method of claim 1, wherein the sensing filmcomprises a hydrophilic material.
 11. The method of claim 1, wherein thesensing film comprises palladium.
 12. The method of claim 1, wherein thesensing film comprises graphene.
 13. The method of claim 1, wherein thesensing film comprises a carbon nanotube array.
 14. The method of claim1, wherein the sensor has a maximum dimension of less than 2 mm². 15.The method of claim 3, wherein the sensor layer and antenna layer areprinted on opposing sides of the piezoelectric substrate layer.
 16. Themethod of claim 15, wherein at least a portion of the sensor layer andthe antenna layer are printed simultaneously.